The major program concerns in vitro growth and calcification of embryonic bone. Femurs of chick embryos in the presence of heparin increased in size, had reduced dry weight, and showed relatively little change in calcium content as compared with controls. Effects of heparin on bone martrix and marrow cells are being examined. Preliminary results indicate that the calcium content of embryonic chick femurs is reduced by cobalt in concentrations of 100 micron M and 200 micron M. The effects of cobalt on cell division, calcium deposition and turnover, and ultrastructure of marrow cells will be studied in embryonic bone. An epithelial membrane preparation, one cell in thickness, from molluscan mantle is being used to investigate transport of calcium and bicarbonate and the role of carbonic anhydrase in transport.